Elaborating, Nomura goes on to explain that the Final Fantasy characters were used in the earlier games in the series as something of a shortcut - familiar faces to help get fans 'on board' with the series and comfortable with the original cast of characters present in Kingdom Hearts.
"I felt nobody really knew these new characters, so it was harder for them to stand their ground just yet. And so, we had a lot of Final Fantasy characters involved to lend a hand for everyone to get to know these [original Kingdom Hearts] characters better," Nomura notes.
"Now, there are so many original characters from Kingdom Hearts that are so well-loved, and people want to see more of those characters. With Kingdom Hearts III, since we did have so many original Kingdom Hearts characters, it was hard to find room for including more Final Fantasy characters."